1. Field of Invention
The present invention relates to a type of rotary electrical device of the type known as a synchronous machine, and in particular to a synchronous machine for use in a vehicle motor-generator apparatus that is controllable for selectively performing electric motor and electric power generation functions.
2. Description of Related Art
There have been proposals made in the prior art to use a synchronous machine of the type having a field winding in a motor-generator apparatus of a vehicle which is driven by an internal combustion engine (referred to in the following simply as the engine), so that a single rotary electrical device can be employed as a motor for the purpose of starting the engine and also as a generator (driven from the engine) for supplying electric power for charging the vehicle battery while the engine is running. Alternatively, such a vehicle motor-generator apparatus could also be utilized to provide electrically driven motive power when required, in the case of a hybrid type of motor vehicle.
However with such an apparatus, a worst-case operating condition occurs when the engine is halted immediately after the vehicle motor-generator apparatus has been running in a condition in which it is generate a substantial level of electric power (so that the synchronous machine is at a high temperature), and the engine is then restarted shortly after having been halted, so that a high level of current must be passed through the field winding of the synchronous machine under a condition of low speed of rotation. As a result, the maximum allowable temperature of the field winding may be exceeded, so that insulation breakdown or deterioration may occur. Such a condition may occur relatively frequently for example when the vehicle operates in an xe2x80x9cidling haltxe2x80x9d mode, whereby the engine is halted automatically under certain conditions.
It is necessary to design such a vehicle motor-generator apparatus such that the maximum allowable temperature of the field winding will not be exceeded even under the worst-case condition described above, when operating under the maximum anticipated ambient temperature. For that reason, it has been necessary for the size and weight of the synchronous machine used in such a vehicle motor-generator apparatus to be substantially greater than that of a conventional vehicle generator, having only an electric power generating function.
However it is undesirable to have to design such a vehicle motor-generator apparatus on the basis of thermal conditions which occur when the apparatus must supply a large amount of torque during a very short time interval, for the purpose of starting the vehicle engine, since in that case the thermal capacity of the vehicle motor-generator apparatus will be greatly in excess of that which is necessary during the majority of the time that the apparatus is operated, i.e., the configuration of the synchronous machine of the vehicle motor-generator apparatus will be excessively large and heavy.
Furthermore an important parameter of such a vehicle motor-generator apparatus is the time interval which must elapse, after the vehicle engine has been halted, between the issuance of an xe2x80x9cengine startxe2x80x9d command to the control section of the vehicle motor-generator apparatus (i.e., when the ignition switch of the vehicle is actuated) and the time point at which the engine then actually is started. In the prior art, the problem exists that the field winding of the synchronous machine of the vehicle motor-generator apparatus has a substantial amount of inductance, so that a significant amount of time is required for current build-up to occur in the field winding to achieve sufficient torque to initiate engine starting, and this increases the amount of time required to effect starting of the engine.
One objective of the present invention therefore is to provide a vehicle motor-generator apparatus whereby the size and weight of the synchronous machine used in the vehicle motor-generator apparatus can be reduced by comparison with the prior art, while a further objective is to reduce the amount of time required to perform engine starting, by such a vehicle motor-generator apparatus.
Moreover, in Japanese patent HEI 8-214470 a field winding type of synchronous machine is described whereby a phase-advanced AC current, supplied from a power inverter (i.e., a DC-AC and AC-DC converter), is caused to flow in the armature winding of the synchronous machine when the synchronous machine operates to generate electric power. This enables a higher level of generated electric power to be attained when the synchronous machine is driven at a low speed of rotation, by comparison with a conventional type of vehicle generator apparatus which applies only DC rectification to the AC output voltage from the synchronous machine.
Furthermore, such a method can also enable increased efficiency of electric power generation to be achieved when the synchronous machine is driven at a high speed of rotation, by utilizing field control.
However in the prior art it has been necessary to provide such devices as power MOS transistors as switching elements, for supplying the phase-advanced current to the armature winding of the synchronous machine. As a result, the circuit cost becomes higher than in the case of an apparatus which uses only diode rectifiers. Furthermore due to the use of phase-advanced current supply to the armature winding of the synchronous machine, a lowering of efficiency occurs because of increased resistive losses in the armature winding and increased generation of heat, so that the requirements for cooling the armature winding become more stringent.
Furthermore in Japanese patent HEI 10-30463, a synchronous machine is disclosed which is of the type having a field winding and having permanent magnets embedded in the rotor, whereby the effective magnetic flux in the rotor can be controlled such as to eliminate the need to produce a weak flow of current in the armature winding when the synchronous machine is operated at a high speed of rotation, i.e., a flow of current for the purpose of preventing an excessively high level of voltage being generated due to the magnetic fields of the permanent magnets under such a condition of high speed of rotation. Furthermore, with that synchronous machine, even if armature current control becomes ineffective when the synchronous machine is operated at a high speed of rotation, the magnetic flux of the permanent magnets is shunted in such a way that high stability is ensured. That type of synchronous machine will be referred to in the following as a combination permanent magnet and field winding synchronous machine of magnetic shunt type.
However with such a prior art combination permanent magnet and field winding synchronous machine of magnetic shunt type, since the magnetic circuit is complex, there is a high degree of magnetic reluctance in the magnetic circuit, so that the device becomes large in scale in relation to its electric power generating capability.
It is therefore another objective of the present invention to overcome the above disadvantage, by providing a combination permanent magnet and field winding synchronous machine of magnetic shunt type, for use in a vehicle motor-generator apparatus, which maintains a high degree of suppression of the adverse effects of magnetic flux during operation at a high speed of rotation, while the levels of generated torque and output power are increased in relation to the size of the machine. The suitability of such a synchronous machine for use in a vehicle motor-generator apparatus can thereby be increased.
With the present invention, the various objectives summarized above are attained as follows.
According to a first aspect, the invention provides a vehicle motor-generator apparatus comprising a field winding type of synchronous machine, an AC-to-DC and DC-to-AC power converter which converts the DC voltage of a battery of the vehicle to an AC voltage, to thereby supply an AC armature current to an armature winding of the synchronous machine, a field current supply circuit for supplying a field current to the field winding of the synchronous machine, to produce a field winding magnetic flux, and a control circuit for controlling the AC-to-DC and DC-to-AC power converter and the field winding circuit, wherein while the synchronous machine is being operated as an electric motor to perform starting of the engine of the vehicle, the AC-to-DC and DC-to-AC power converter supplies to the armature winding the armature current as a current having a component which forms a magnetic flux in the same direction as that of the field winding magnetic flux. The rotor of the synchronous machine may be provided only with a field winding, or may be of a type which is provided with both a field winding and a plurality of permanent magnets fixedly attached to the rotor, arranged such as to produce magnetic fields of successively alternating polarity around the periphery of the rotor.
Such an apparatus may be further configured such that during an initial period of an engine start-up interval, the AC-to-DC and DC-to-AC power converter supplies to the armature winding the armature current having a component which forms a magnetic flux in the same direction as that of the field winding magnetic flux, and thereafter operates to set the phase of the armature current such as to enhance the generation of torque by the synchronous machine for starting the engine.
Alternatively, such an apparatus can be configured such that when the engine is being started, prior to current being supplied to the field winding, the armature winding is supplied with an armature current such that an armature current-induced magnetic flux is formed which is oriented at approximately the same angular position as that of the field winding magnetic flux.
According to another aspect of the apparatus, designating a maximum allowable temperature of the rotor of the synchronous machine as Tmax, a maximum temperature that will be attained by the rotor during electric power generation operation as Tgmax, and a thermal capacity of the rotor as Q, and also designating T as the duration of an engine starting operation, the resistance of the field winding as r, and the field current as i, the control circuit is configured to limit the field current (during both the electric power generating mode and the electric motor mode, i.e., engine starting) to a value such that (Tgmax+(i2.r.t)/Q) is lower than the temperature value Tmax, preferably by an amount that is within the range 20xc2x0 C. to 40xc2x0 C. It can thereby be ensured that the insulation of the field winding of the synchronous machine will not be destroyed due to operation at an excessive temperature, while reducing the size and weight of the synchronous machine to the greatest possible degree, by enabling reduction of the value of Q.
According to another aspect, the control circuit can be configured to derive an electrical quantity relating to an average value of the field current and an average value of the armature current during a predetermined time interval which extends up to the commencement of an engine starting operation effected by the motor-generator apparatus, and to limit the field current or the armature current to a value that is determined based upon the electrical quantity, during the electric power generation mode. The maximum amount of field current which can be supplied to the rotor of the synchronous machine during engine starting operation must be, as described above, limited such as to ensure that the rotor temperature does not rise to a level at which destruction of the insulation material of the field winding may occur. For any particular value of field current that is supplied during engine starting, the level to which the rotor temperature will rise (by the end of the engine starting interval) is based on the rotor temperature at the start of the engine starting operation. That initial rotor temperature is strongly affected by the average values of field current and armature current which were flowing during electric power generation operation immediately prior to commencing the engine starting operation. With this aspect of the invention, these average values of current are utilized as a basis for determining the maximum value of field current that can be supplied during the engine starting operation (and hence, the level of torque that can be generated by the synchronous machine during the engine starting operation).
Alternatively, the control circuit can be configured to derive an electrical quantity relating to ambient temperature, and to control the field current, during electric power generation operation or during engine starting operation, to a value which is based upon that electrical quantity. Since the temperature which the rotor will have attained by the end of the engine starting interval is strongly affected by the rotor temperature at the start of the engine starting operation, it will be understood that such a form of control of the field current can also enable the value of field current supplied during the engine starting operation to be increased, consistent with ensuring that thermal damage to the field winding insulation material will not occur.
The invention therefore discloses various means whereby the size and weight of the synchronous machine of such a vehicle motor-generator apparatus can be minimized, consistent with preventing destruction of the field winding insulation and with achieving satisfactory engine starting and electric power generating performance.
According to another aspect, in which the rotor of the synchronous machine has a rotor core with the field winding wound thereon, the control circuit is configured to function during an engine starting operation such as to control the field current and the armature current in a manner whereby magnetic saturation occurs in a magnetic circuit which passes through the rotor core, and to control the value of a field magnetic force Ff (i.e., the product of the field current by the number of turns of the field winding) to a higher value than an armature magnetic force Fa (i.e., the product of the armature current by a number of turns of the armature winding). In that way it becomes possible to reduce the electrical losses which occur during the electric power generating mode of operation, and thereby achieve increased efficiency.
According to another aspect, one or more layers of thermally conductive film are disposed between layers of the field winding, extending along a winding direction of the field winding and in contact with the rotor core. In that way, heat generated within the field winding can be efficiently transferred to the rotor core and thereby rapidly dissipated, thereby enabling a higher level of field current to be applied during engine starting operation, with resultant increased torque. Thus, the rotor (and hence, the overall synchronous machine) can be made smaller and lighter than in the prior art.
According to another aspect, the control circuit is configured such that during a predetermined initial period of an engine starting interval, the control circuit supplies a high level of field current to the field winding (e.g., with the aforementioned duty ratio at its maximum value of 100%), in order go generate sufficient torque to overcome the initial static friction of the engine and rotate the engine through the first compression cycles, and thereafter supplies the field current with a reduced value of duty ratio during the remaining part of the engine starting interval. Preferably, this is performed such that the aforementioned initial period of supplying the maximum level of field current is of sufficient duration to encompass at least the first compression stroke of the engine, as it is rotated by the synchronous machine.
After that initial period, the level of field current is reduced to a level which will produce sufficient torque to continue rotation of the engine, and this level of field current is maintained until engine starting has been completed. In that way, it becomes possible to use a smaller size of synchronous machine to perform engine starting, by comparison with the prior art in which a large amount of torque (sufficient to overcome the static friction forces and rotate the engine through the initial compression strokes) is continuously applied during the entire engine starting interval. With such a prior art apparatus, due to considerations of the heat generated in the drive motor when continuously supplying a high level of field current during a relatively long period of time, it is necessary to use a large and heavy device to perform the starter motor function. However with the present invention, that problem can be overcome as described above. After the level of field current has been reduced, that level can be continuously supplied during a relatively long interval, with a minimal amount of heat being generated in the field winding, until it is ensured that engine starting has been achieved. In that way, the performance requirements for the synchronous machine, when functioning as an engine starter motor, can be substantially reduced, so that a small and light device can be utilized.
According to another aspect, the invention provides a vehicle motor-generator apparatus based on a field winding type of synchronous machine, in which the rotor has a rotor core of cylindrical form, with an even number of permanent magnets respectively retained in an even number of magnet accommodation apertures each formed extending along an axial direction in the stator core, with an even number of field poles of the permanent magnets being arranged successively alternating in polarity around the outer periphery of the stator core. The rotor core further includes magnetic shunt members, i.e. pins formed of a soft magnetic material, inserted in respective apertures extending along the axial direction of the rotor core, for shunting the magnetic fields of the permanent magnets, and disposed such that the magnetic flux of the field winding passes along these pins, in the axial direction of the rotor. The synchronous machine further includes a yoke member, disposed at the inner periphery of the rotor core, which acts in conjunction with the rotor core and the magnetic shunt members (pins) to form a flow path for the magnetic flux produced by the field winding. It is a basic feature of such a synchronous machine that each of the permanent magnets has at least a portion thereof which is positioned radially inward in relation to the magnetic shunt members, and that the rotor core has a magnetic path which extends from an external peripheral position between two circumferentially adjacent ones of the 90field poles and between two adjacent ones of the permanent magnets to a position which is radially inward from the permanent magnets.
As a result, the synchronous machine can generate reluctance torque under a condition in which saturation of magnetic paths within the rotor core occurs under a condition of high levels of current flow in the armature and field windings.
According to another aspect, the vehicle motor-generator apparatus is of a type in which a phase-advanced AC current is supplied to the armature winding of the synchronous machine during electric power generation operation, to increase the level of electric power that can be produced., i.e., with the phase-advanced current being advanced in phase by a predetermined phase angle with respect to the AC voltage produced from the armature winding as a result of relative motion of the armature winding through the magnetic field produced by the rotor. With the present invention, the synchronous machine is formed with a rotor-side iron core portion (i.e., providing a magnetic path, within the rotor, for the magnetic flux induced by current flow through the field winding) which has a smaller value of magnetic reluctance in a direction that is at right angles to the direction of that magnetic flux than in the direction of the magnetic flux.
As a result, the electrical generation power factor of the synchronous machine is increased, i.e., the total amount of current which flows through the armature winding (for a specific level of generated electric power) is reduced, by comparison with prior art types of apparatus which utilize the method of supplying a phase-advanced current to the armature winding. Hence, the scale of the synchronous machine, as regards electric power generation requirements, can be reduced, and manufacturing costs lowered.